We have developed a prototype of a multichannel electrode system for recording neural spikes and slow wave activity in acute and chronic vertebrate preparations. From the experience of our initial success with problems in cat visual cortex, we propose modifications that will considerably enhance the system's performance. The modified multichannel electrode will have 22 recording sites and 2 lesion site spanning 2.4 mm and spaced at 100 um intervals along its edge. By the use of integrated circuit technology, the electrode will be built in layers of silicon nitride, platinum, and silicon nitride on a molydenum substrate. The total thickness will only be 20 um and the maximum width at the base of its wedge shape will be 135 um. A modified coonnector will be built that will allow quick replacement of damaged electrodes. It will be small enough for moving preparations, and will contain preamplifiers in a hybrid of IC chips. An amplifier array will also be built to process both unit and slow wave activity. Evaluation of the electrode will be performed by standard electrical tests and by extensive use in vivo. The rat hippocampus is a particularly useful test preparation because of its well studied physiology, distinctive lamination of cellular subpopulations, anatomically separated inputs and outputs, and characteristic slow wave pattern and related unitary activity. Recording performance of the multichannel electrode will be compared with that of a conventional metal microelectrode in 3 measures of neural activity. 1) laminar patterns of field potentials evoked by electrical stimulation of known pathways, 2) laminar amplitude and phase relationships of the theta rhythm, and 3) sensitivity and selectivity for unit activity and fixing patterns in relationship to theta cycle. These evaluations will be carried out in both urethane anesthetized and awake, behaving preparations to examine the generality of the system's usefulness as a new tool for the study of local circuit interactions.